The present invention relates to a touch responsive arrangement for a keyboard controlled electronic musical instrument and more particularly to an apparatus and method for sensing the force with which a key has been struck.
In many prior art electronic keyboard musical instruments, such as electronic organs, pianos, synthesizers and the like, when a playing key is depressed or struck, an electric switch will be closed to indicate to the electronic circuitry that a key has been depressed. It is often desirable in instruments of this type to detect the force with which a key is depressed because the musician, by depressing a key not only selects the particular note to be played by the instrument but also controls the amplitude with which the note is to be played as well as many other musical effects which he may want to create. In many of the prior art electronic pianos, when a key is depressed, a contact member is disconnected from one bus and is then brought into contact with a second bus on full key depression. The time of travel of the contact between the two busses is measured to arrive at a measure of key velocity and the amplitude of the tone is altered in accordance therewith.
In these prior art systems, the force with which the keys are struck is measured as a function of the travel time of the contact from the time that the key is first actuated to the time that the key actuates the note being played. There are several disadvantages to these velocity measuring types of systems. First of all the switch contacts tend to become dirty after a period of time thereby causing extroneous signals and intermittent operation. Secondly there is a problem of switch bounce which is particularly troublesome in digital instruments because digital circuitry detects the switch bounce as repeated key closures whereas only one key closure is intended. Bounce must therefore be overcome by utilizing special switch debouncing circuitry.
Switch contacts can also become deformed so that different switches will close at different points in the depression of their respective keys. In velocity sensitive keyboards, such as those used in electronic pianos, inconsistencies in switch closure times will result with some keys producing tones of different amplitudes for a given key depression velocity, thereby resulting in non-uniform operation.
An additional problem encountered in prior art electronic keyboard instruments is that if a musician does not fully release a key but performs a rapid trill on a particular key by keeping his finger in touch with the key and rapidly successively operating the key a number of times, the prior art timing circuits may never be actuated because timing is generally initiated at the moment the key is first depressed from a released position. Therefore, if the key is not depressed from a key released position, the keyboard key velocity detecting circuitry will not be actuated and no output will be generated by this type of playing. It is therefore desired to provide an electronic musical keyboard instrument wherein the detecting mechanism is sensitive not only to normal playing of the keys but is sensitive to playing of the keys without an actual release of the keys between key actuations.
Another problem with many prior art electronic keyboards has been that uniform and predictable aftertouch control is particularly difficult to achieve when utilizing compressible resistive strips or piezoelectric elements, because their electric characteristics may vary with time, temperature and use. Furthermore electrical changes resulting from physical changes in the element, such as in the case of the amount of compression of a resistive strip, is difficult to predict and control with the consistency that is desirable.
The prior art art keyboard system described in the aforementioned patent, U.S. Pat. No. 4,558,623, comprises a moveable pickup that is positioned within an electrostatic field between two stationary electrodes, and the relative positioning of the moveable electrode within the field produces a variable output voltage. The pickup is moved by depression of the key, and the velocity with which the key is struck is detected by timing the movement of the pickup from the undepressed position to a fully depressed position but before overtravel. Again, a timing technique is used to measure key velocity rather than the terminal velocity of the key, which is more like the action in a conventional acoustic piano. Full release of the key is often necessary before the timing sequence can be activated, so that trills and other rapid playing effects may result in missed notes.